Method of forming a semiconductor device having sub-surface trench charge compensation regions

ABSTRACT

In one embodiment, a semiconductor device is formed having sub-surface charge compensation regions in proximity to channel regions of the device. The charge compensation trenches comprise at least two opposite conductivity type semiconductor layers. A channel connecting region electrically couples the channel region to one of the at least two opposite conductivity type semiconductor layers.

The present application is a divisional application of prior U.S. patent application Ser. No. 11/442,706, filed May 30, 2006 now U.S. Pat. No. 7,679,146, which is hereby incorporated by reference and priority thereto is hereby claimed.

FIELD OF THE INVENTION

This invention relates generally to semiconductor devices, and more specifically to power switching devices and methods of their manufacture.

BACKGROUND OF THE INVENTION

Metal-oxide semiconductor field effect transistors (MOSFETs) are a common type of power switching device. A MOSFET device includes a source region, a drain region, a channel region extending between the source and drain regions, and a gate structure provided adjacent to the channel region. The gate structure includes a conductive gate electrode layer disposed adjacent to and separated from the channel region by a thin dielectric layer.

When a MOSFET device is in the on state, a voltage is applied to the gate structure to form a conduction channel region between the source and drain regions, which allows current to flow through the device. In the off state, any voltage applied to the gate structure is sufficiently low so that a conduction channel does not form, and thus current flow does not occur. During the off state, the device must support a high voltage between the source and drain regions.

Today's high voltage power switch market is driven by two major parameters: breakdown voltage (BVdss) and on-state resistance (Rdson). For a specific application, a minimum breakdown voltage is required, and in practice, designers typically can meet a BVdss specification. However, this is often at the expense of Rdson. This trade-off in performance is a major design challenge for manufacturers and users of high voltage power switching devices.

Recently, superjunction devices have gained in popularity to improve the trade-off between Rdson and BVdss. In a conventional n-channel superjunction device, multiple heavily-doped diffused n-type and p-type regions replace one lightly doped n-type epitaxial region. In the on state, current flows through the heavily doped n-type regions, which lowers Rdson. In the off or blocking state, the heavily doped n-type and p-type regions deplete into or compensate each other to provide a high BVdss. Although superjunction devices look promising, significant challenges still exist in manufacturing them.

Another problem with present high voltage power switch products is that they typically require a large input (e.g., gate or control electrode) charge for switching from one state to another. This requirement places, among other things, an extra burden on peripheral control circuitry.

Accordingly, high voltage power switching device structures and methods of manufacture are needed that provide lower Rdson, high BVdss, and that reduce input charge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an enlarged partial cross-sectional view of a semiconductor device in accordance with the present invention;

FIGS. 2-11 illustrate enlarged partial cross-sectional views of the semiconductor device of FIG. 1 at various stages of fabrication;

FIG. 12 illustrates a highly enlarged partial cross-sectional view of a portion of a semiconductor device according to another embodiment of the present invention; and

FIG. 13 illustrates a highly enlarged partial cross-sectional view of a portion of a semiconductor device according to a further embodiment of the present invention.

For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-channel devices, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with the present invention. For clarity of the drawings, doped regions of device structures are illustrated as having generally straight line edges and precise angular corners. However, those skilled in the art understand that due to the diffusion and activation of dopants the edges of doped regions are generally not straight lines and the corners are not precise angles.

In addition, the device of the present invention may embody either a cellular design (where the body regions are a plurality of cellular regions) or a single body design (where the body region is compromised of a single region formed in an elongated pattern, typically in a serpentine pattern). However, the device of the present invention will be described as a cellular design throughout the description for ease of understanding. It should be understood that it is intended that the present invention encompass both a cellular design and a single base design.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows a partial cross-sectional view of an insulated gate field effect transistor (IGFET), MOSFET, superjunction device, or switching device or cell 10 in accordance with an embodiment of the present invention. By way of example, device 10 is among many such devices integrated with logic and/or other components into a semiconductor chip as part of a power integrated circuit. Alternatively, device 10 is among many such devices integrated together to form a discrete transistor device.

Device 10 includes a region of semiconductor material 11, which comprises for example, an n-type silicon substrate 12 having a resistivity in range of approximately 0.001 to about 0.005 ohm-cm, and may be doped with arsenic. In the embodiment shown, substrate 12 provides a drain region for device 10, which is coupled to conductive layer 13. A semiconductor layer 14 is formed in or on substrate 12, and in accordance with the present invention is n-type or p-type and doped light enough so as to not impact charge balance in the trench compensation regions described below. In one embodiment, layer 14 is formed using conventional epitaxial growth techniques. In an embodiment suitable for a 600 volt device, layer 14 is doped n-type or p-type with a dopant concentration of about 1.0×10¹³ atoms/cm³ to about 1.0×10¹⁴ atoms/cm³ and has a thickness on the order of about 40 microns to about 60 microns. The thickness of layer 14 is increased or decreased depending on the desired BVdss rating of device 10. In an alternative embodiment, semiconductor layer 14 comprises a graded dopant profile with semiconductor layer 14 having a higher dopant concentration in proximity to substrate 12, and transitioning either gradually or abruptly to a lower concentration for the balance of its thickness. Other materials may be used for body of semiconductor material 11 or portions thereof including silicon-germanium, silicon-germanium-carbon, carbon doped silicon, III-V materials, or the like.

In accordance with the present invention, device 10 further includes a filled trench structure or structures 510 formed in region of semiconductor material 11. Filled trench structure 510 comprises a superjunction portion, sub-surface charge compensation portion, or compensation portion 22, and a control portion or gate control portion 511 overlying compensation portion 22. Compensation portions 22 comprise spaced apart and sub-surface filled trenches, semiconductor material filled trenches, epitaxial filled regions or trenches, charge compensating trench regions, deep trench charge compensation regions, charge compensating filled trenches, charge compensating portion or charge compensation regions 22. Portions 22 include a plurality of layers or multiple layers of semiconductor material, including layers of opposite conductivity type, which are preferably separated by an intrinsic or buffer semiconductor layer or layers. The intrinsic layer functions, among other things, to prevent or reduce intermixing of the opposite conductivity type layer (i.e., the two charge layers), which is believed to negatively impact the conduction efficiency of device 10 in the on state. As used herein, charge compensation generally means that the total charge of the opposite conductivity type layers is substantially balanced or equal.

In one embodiment, compensation portions 22 include multiple layers or stacked layers of semiconductor material formed using single crystal (i.e., not polycrystalline) epitaxial growth techniques, and these layers terminate or end below major surface 18 a distance 181. For example, compensation portions 22 include a p-type layer 23 formed on, over, or adjoining the trench walls or surfaces adjacent to body of semiconductor material 11. An intrinsic semiconductor or buffer layer 24 is formed on, over, or adjoining p-type layer 23, an n-type layer 26 is formed on, over, or adjoining intrinsic semiconductor layer 24, and an intrinsic semiconductor or buffer layer 27 is formed on, over, or adjoining n-type layer 26. Intrinsic layer 24 functions, among other things, to prevent or reduce the mixing of dopants from layers 23 and 26, which, as stated previously, improves the conduction efficiency of device 10. Intrinsic layer 27 functions, among other things, to fill or partially fill the trench. For an n-channel device and in accordance with the present invention, n-type layers 26 provide a primary vertical low resistance current path from the channel to the drain when device 10 is in an on state. When device 10 is an off state, p-type layers 23 and n-type layers 26 compensate each other in accordance with the present invention to provide an increased BVdss characteristic. It is understood that additional n-type and p-type layers may be used, and preferably separated by additional intrinsic or buffer layers. In an alternative embodiment and as shown in FIG. 1, a dielectric layer 28 is formed overlying the outermost (e.g., layer 26 or 27). In one embodiment, dielectric layer 28 fills any remaining space within compensation portion 22. In another embodiment dielectric layer 28 only partially fills any remaining space within portion 22 leaving, for example an air gap. By way of example, dielectric layer 28 comprises an oxide, a nitride or combinations thereof. In another embodiment, dielectric layer 28 comprises a thin thermal oxide capped with a thin polysilicon layer followed by a deposited TEOS layer. It was observed that in some applications, the thin oxide capped with polysilicon reduces shear stress from the deposited oxide thereby improving device performance. It is further understood that during thermal processing, n-type and p-type dopants from layers 26 and 23 diffuse into the buffer layers, and that distinct buffer layers may or may not be present in the final device. However, when deposited or formed, buffer layers 24 and/or 27 have a lower dopant concentration than layers 23 and 26.

By way of example, p-type layers 23 and n-type layers 26 each have a dopant concentration on the order of about 9.0×10¹⁶ to about 3.0×10¹⁶ atoms/cm³, and each have a thickness of about 0.1 microns to about 0.3 microns respectively. In one embodiment, intrinsic semiconductor or buffer layers 24 and 27 are undoped or very lightly doped p-type with a dopant concentration of less than about 1.0×10¹⁴ atoms/cm³, and each has a thickness of about 0.5 microns to about 1.0 microns. The thickness of layer 27 is adjusted, for example, to fill the balance of the trench between the deposited layers if dielectric layer 28 is not used.

In accordance with the present invention, dopant from p-type layer 23 is diffused into semiconductor layer 14 to form p-type regions or laterally doped or diffused regions 231 (represented as dashed lines) underneath body regions 31 described below. P-type regions 231 laterally diffusing from adjacent compensation portions 22 may either completely merge together, or may not completely merge as shown in FIG. 1 so that a portion of semiconductor 14 is still present in the finished device. That is, the actual diffusion distance between adjacent laterally diffused region 231 is variable.

In accordance with the present invention, in one embodiment, diffused regions 231 comprise the opposite conductivity type to that of semiconductor layer 14. This embodiment provides for a unique structure where both the active device structure and edge termination structures (not shown) are formed in the same layer (i.e., layer 14), but the active device (i.e., device 10) is in a p-type layer because of laterally diffused regions 231, and the edge termination structures are formed in n-type layer 14 laterally separated from compensation portions 22.

Although not shown, it is understood that during the formation of device 10, n-type dopant from highly doped substrate 12 diffuses into the lower portions of compensation regions 22 so that those portions of compensation regions 22 that are within substrate 12 become more heavily doped n-type.

Trench gate structures or control portions 511 include a control or gate electrode or conductive layer or region 57, separated on vertical sidewalls of the trench regions by a gate dielectric layer, region or material 43. In one embodiment, gate dielectric layer 43 comprises a silicon oxide, and has a thickness of about 0.05 microns to about 0.1 microns. In alternative embodiments, gate dielectric layer 43 comprises silicon nitride, tantalum pentoxide, titanium dioxide, barium strontium titanate, or combinations thereof including combinations with silicon oxide, or the like. Conductive gate regions 57 comprise, for example, n-type polysilicon, and are about 0.3 microns to about 0.5 microns in thickness.

In accordance with one embodiment of the present invention, an optional thick dielectric layer 431 separates, isolates, or insulates gate conductive regions 57 from sub-surface trench compensation regions 22. In this embodiment, dielectric layer 431 is thicker than dielectric layers 43. By way of example, dielectric layer 431 comprises about 0.1 microns to about 0.2 microns of thermal oxide. In an alternative embodiment, gate dielectric layer 43 is used to isolate gate conductive regions 57 from sub-surface trench compensation regions 22.

A body or doped region 31 is formed in semiconductor layer 14 between and in proximity to or adjacent to, or adjoining trench gate structures 510, and extends from major surface 18 of body of semiconductor material 11. In one embodiment, body regions 31 comprise p-type conductivity, and have a dopant concentration suitable for forming an inversion layer that operates as conduction channels 45 of device 10 when gate structures 510 are appropriately biased as described below. Body regions 31 extend from major surface 18 to a depth of about 1.0 to about 5.0 microns. N-type source regions 33 are formed within or in body region 31 in proximity to or adjacent to, or adjoining trench gate structures 510. In one embodiment, source regions 33 extends from major surface 18 to a depth of about 0.2 microns to about 0.5 microns. One or more p-type body contact regions 36 are formed in body region 31 partially within and/or below source regions 33. Body contact regions 36 are configured to provide a lower contact resistance to body region 31, and to lower the sheet resistance of body regions 31 under source regions 33, which suppresses parasitic bipolar effects.

In accordance with the present invention, device 10 further includes n-type channel connect, or drain extension regions 32, which are configured to electrically couple channel regions 45 to sub-surface trench compensation regions 22. In one embodiment and as shown in FIG. 1, channel connect regions 32 are formed below and adjoining body regions 31. Channel connect regions 32 further adjoin upper surfaces of or portions of layers 23 and 26 to provide a conduction path between source regions 33 and layer 26 when device 10 is in operation.

An interlayer dielectric region 48 is formed over lying major surface 18, and comprises for example, a first dielectric layer 51 formed overlying conductive gate regions 57, and a second dielectric layer 61 formed overlying first dielectric layer 51. By way of example, dielectric layer 51 comprises a silicon oxide, and has thickness from about 0.02 microns to about 0.05 microns. Dielectric layer 61 comprises for example, a deposited oxide, and has a thickness of about 0.4 microns to about 1.0 microns.

Openings are formed in interlayer dielectric region 48 to provide contacts to device 10 for source contact layer 63. As shown, a portion of major surface 18 is etched so that source contact layer 63 makes contact to both source regions 33 and body regions 36. In one embodiment, source contact layer 63 comprises a titanium/titanium nitride barrier layer and an aluminum silicon alloy formed overlying the barrier layer, or the like. Drain contact layer 13 is formed on an opposing surface of region of semiconductor material 11, and comprises, for example, a solderable metal structure such as titanium-nickel-silver, chrome-nickel-gold, or the like.

The operation of device 10 proceeds as follows. Assume that source terminal 63 is operating at a potential V_(S) of zero volts, gate regions 57 receive a control voltage V_(G)=5.0 volts, which is greater than the conduction threshold of device 10, and drain terminal 13 operates at drain potential V_(D)=5.0 volts. The values of V_(G) and V_(S) cause body region 31 to invert under gate regions 57 to form channels 45, which electrically connect source regions 33 to channel connect regions 32. A device current I_(D) flows from drain terminal 13 and is routed through n-type layers 26, channel connect regions 32, channels 45, source regions 33, to source terminal 63. Hence, current I_(D) flows vertically through n-type layers 26 to produce a low on resistance. In one embodiment, I_(D)=1.0 amperes. To switch device 10 to the off state, a control voltage V_(G) of less than the conduction threshold of device is applied to gate regions 57 (e.g., V_(G)<5.0 volts). This removes channels 45, I_(D) no longer flows through device 10. In the off state, n-type layers 26 and p-type layers 23 compensate each other as the depletion region from the primary blocking junction spreads, which enhances BVdss.

Turning now to FIGS. 2-9, a process for forming device 10 in accordance with the present invention is described. FIG. 2 shows an enlarged partial cross-sectional view of device 10 at an early stage of fabrication. An example of the material characteristics of body of semiconductor material 11 was provided in conjunction with FIG. 1 above. In an early step, a first dielectric layer 40 is formed over major surface 18, and comprises for example, a silicon oxide about 0.05 microns to about 0.1 microns thick. A standard photolithography step is then used to provide openings for p-type body regions 31 and edge termination structures (not shown). P-type body regions 31 are selectively formed in semiconductor layer 14 through dielectric layer 40. In an embodiment suitable for a 600 volt device, boron is implanted at a dose of about 1.0×10¹³ atoms/cm² and an implant energy of about 160 KeV to form regions 31. A second dielectric layer 44 comprising for example a different material than first dielectric layer 40 is then formed overlying first dielectric layer 40. By way of example, second dielectric layer 44 comprises a silicon nitride when first dielectric layer 40 comprises a silicon oxide. In one embodiment, second dielectric layer 44 comprises approximately 0.2 microns of silicon nitride, and is formed using conventional deposition techniques. Next, the implanted p-type dopant is heat treated to diffuse the dopant to a desired depth to form regions 31. By way of example, body regions 31 have a depth of about 3.0 to about 5.0 microns.

FIG. 3 shows an enlarged partial cross-sectional view of device 10 at a subsequent stage of fabrication. Hard mask layer 71 is formed overlying major surface 18 and patterned to form openings 72 through hard mask layer 71, second dielectric layer 44, and first dielectric layer 40 to expose portions of major surface 18. By way of example, hard mask layer 71 comprises about 1.0 microns of deposited oxide. By way of example, openings 72 have a width 74 on the order of about 3.0 microns to about 5.0 microns. Next, trenches 122 are formed through semiconductor layer 14. In one embodiment, trenches 122 extend into at least a portion of substrate 12. The depth of trenches 122 is determined by the thickness of semiconductor layer 14, which is a function of BVdss. In one embodiment, Deep Reactive Ion Etching (DRIE) etching with a fluorine or chlorine based chemistry is used to form trenches 122. Several techniques are available for DRIE etching including cryogenic, high density plasma, or Bosch DRIE processing. In one embodiment, trenches 122 have substantially vertical sidewalls. In an alternative embodiment, trenches 122 have a tapered profile where the width of the trench at the trench lower surface is less than width 74. Although trenches 122 are stated as plural, it is understood that trenches 122 may be a single continuous trench or connected trench matrix. Alternatively, trenches 122 may be a plurality of individual trenches with closed ends and separated by portions of body of semiconductor material 11.

FIG. 4 shows an enlarged partial cross-sectional view of device 10 at a further stage of processing. At this point, layers of semiconductor material are formed, grown, or deposited in trenches 122 as a first stage in forming filled trenches or sub-surface charge compensation regions 22. In one embodiment, single crystal semiconductor epitaxial growth techniques are used to fill trenches 122. That is, single crystal semiconductor layers are grown within trenches 122.

In a first step, a thin thermal oxide (not shown) is formed on the sidewalls of trenches 122 to remove any surface damage caused by the DRIE step. The thin thermal oxide is then removed using conventional isotropic etching techniques (e.g., 10:1 wet oxide strip). Next, body of semiconductor material 11 is placed into an epitaxial growth reactor and pre-cleaned as a first step of the epitaxial growth process. When silicon is the selected semiconductor material for the fill layers (e.g., layers 23, 24, 26, and 27), silicon source gases such as SiHCl₃, SiH₂Cl₂, SiH₄ or Si₂H₆ are suitable for forming these layers. In the embodiment shown, blanket layers are grown (i.e., the layers are grown over major surface 18 in addition to trenches 122). In an alternative embodiment, selective epitaxial growth techniques are used to form layers 23, 24, 26, and 27 so that these layers are not formed overlying major surface 18, but only within trenches 122.

P-type layer 23 is grown first along the surfaces of trenches 122, with boron being a suitable dopant source. By way of example, p-type layer 23 has a dopant concentration on the order of about 3.0×10¹⁶ to about 9.0×10¹⁶ atoms/cm³, and a thickness of about 0.1 microns to about 0.3 microns. In an optional embodiment and as shown in FIG. 4, an intrinsic layer 233 is formed overlying p-type layer 23, and has a thickness of about 0.1 to about 0.2 microns. A capping layer 234 is then formed overlying layer 233, and comprises for example, about 0.05 microns of thermal oxide and about 0.1 microns of nitride. Next, device 10 is heated primarily to laterally diffuse p-type dopant from layer 23 into semiconductor layer 14 to form laterally diffused p-type regions 231. Layer 234 is configured to cap p-type layer 23 during the heat treatment step to prevent dopant from out-diffusing from layer 23. Also, during the heat treatment step, n-type dopant from substrate 12 diffuses into portions 1200 of layer 23 converting portions 1200 to n-type. Further, p-type dopant in layer 23 diffuses into intrinsic layer 233 converting intrinsic layer 233 into p-type layer 23, which is shown as a continuous layer 23 in FIGS. 5-12. After the heat treatment step, capping layer 234 is removed.

Turning now to FIG. 5, intrinsic or buffer layer 24 is grown overlying p-type layer 23, and is either undoped, or is very lightly doped p-type with a dopant concentration of less than about 2.0×10¹⁴ atoms/cm³. Layer 24 has a thickness of about 0.5 microns to about 1.5 microns. N-type layer 26 is then grown overlying layer 24, with a phosphorous, arsenic or antimony dopant source being suitable. In one embodiment, n-type layer 26 has a dopant concentration on the order of about 3.0×10¹⁶ to about 9.0×10¹⁶ atoms/cm³, and a thickness of about 0.1 microns to about 0.3 microns. Intrinsic or buffer layer 27 is then grown over n-type layer 26, and is either undoped (except for those trace impurities typically present in the silicon source material and/or residual dopant gases remaining in the reactor chamber after the previous growth step), or is very lightly doped n-type with a dopant concentration of less than about 2.0×10¹⁴ atoms/cm³. Layer 27 has a thickness of about 0.1 microns to about 0.3 microns. Next a thin wet oxide is grown over layer 27 followed by the formation of dielectric layer 28, which comprises for example a deposited oxide having a thickness suitable to fill trenches 122. In one embodiment, multiple steps are used to form dielectric layer 28, with etch-back or planarization steps done in between deposition steps to ensure that trenches 122 are filled to a desired level. It should be understood that the thicknesses of layers 23, 24, 26, 27, and 28 are adjusted depending on the width of trenches 122.

FIG. 6 shows an enlarged partial cross-sectional view of device 10 at a still further stage of fabrication after layers 28, 27, 26, 24, 23, are planarized and recessed below major surface 18 to form sub-surface filled trench compensation regions or compensation portions 22. In one embodiment, layers 28, 27, 26, 24, and 23 are recessed a distance 181, which is greater than the depth of body regions 131. By way of example, etch back is used to planarize and recess these layers. By way of example, a dry etch techniques with fluorine or chlorine based chemistries are used to etch the layers. In one embodiment, a polysilicon layer and a photoresist layer are first formed overlying dielectric layer 28, and the layers are then etched back or planarized using second dielectric layer 44 as a stop layer to protect portions of major surface 18. In one embodiment and as shown in FIG. 6 portions of sidewalls 228 are etched to be laterally recessed underneath portions 229 of dielectric layer 40 so that upper portions of trenches 122 are wider than the lower portions containing layers 23, 24, 26, 27 and 28, which among other things provides enhanced alignment of channel connecting regions 32 described below.

FIG. 7 shows an enlarged partial cross-sectional view of device 10 after additional processing. A dielectric layer 113 is formed on exposed surfaces of trenches 122 including upper surfaces of layers 23, 24, 26 and 27. By way of example the dielectric layer comprises about 0.1 microns of thermal oxide. Next, channel connecting regions 32 are formed adjoining layers 23 and 24 as well as body regions 32. By way of example, channel connecting regions 32 are formed using ion implantation with an n-type dopant such as phosphorous. An implant dose of about 1.0×10¹³ atoms/cm² to about 1.0×10¹⁴ atoms/cm² and an implant energy of about 120 KeV to about 150 KeV is suitable for one embodiment of the present invention. In one embodiment angled implantation is used to provide lateral penetration of dopant below body regions 31. After the implant step, the dielectric layer 113 is removed using conventional techniques.

FIG. 8 shows an enlarged partial cross-sectional view of device 10 after further processing. Gate dielectric layer 43 is formed overlying exposed surfaces of trenches 122 and connecting regions 32. In one embodiment, gate dielectric layer 43 comprises silicon oxide, and has a thickness of about 0.05 microns to about 0.1 microns. When optional thick dielectric layer 431 is used, the following description illustrates one method of its formation together with FIG. 8. After gate dielectric layer 43 is formed, a polysilicon layer about 0.05 microns thick is formed overlying gate dielectric layer 43. The polysilicon layer is then etched to form poly spacers 333 within an along upper sidewalls of trenches 122 and preferably recessed under dielectric layer 44. A dielectric layer is then formed overlying gate dielectric layer 43 and spacers 333. By way of example the dielectric layer comprises about 0.05 microns of silicon nitride, which is then etch to form nitride spacers 334 adjoining poly spacers 333 and dielectric layers 40 and 44. Next, dielectric layer 431 (shown in FIG. 9) is grown above channel connecting regions 32 between spacers 334. By way of example, dielectric layer 431 comprise about 0.1 to about 0.2 microns of thermal oxide. Nitride spacers 334 and poly spacers 333 are then removed.

FIG. 9 shows an enlarged partial cross-sectional view of device 10 after still further processing. A conductive layer such as a doped polysilicon layer is deposited overlying gate dielectric layer 43 and patterned to form gate conductive regions 57 within trenches 122 above sub-surface trench compensation regions 22. For example, gate conductive regions 57 comprise about 0.3 microns to about 0.5 microns of phosphorous doped polysilicon. In one embodiment, gate conductive regions 57 are annealed prior to etch. Gate conductive regions 57 and gate dielectric region 43 form control portion 511 of filled trench structure 510. In one embodiment, dielectric layer 40 is removed at this point using conventional techniques.

FIG. 10 shows an enlarged partial cross-sectional view of device 10 at another stage of fabrication. A dielectric layer 51 is deposited overlying major surface 18. By way of example, layer 51 comprises a thin oxide layer having a thickness on the order of about 0.02 microns to about 0.07 microns. Next a conventional photolithographic step is used to provide openings for source regions 33. By way of example, a phosphorous implant dose of 3.0×10¹⁵ atoms/cm² with an implant energy of 80 KeV is used for forming source regions 33. Any masking materials such as photoresist used to form source regions 33 are then removed.

FIG. 11 shows an enlarged partial cross-sectional view of device 10 after additional processing. Passivation or dielectric layer 61 is formed overlying major surface 18. By way of example, layer 61 comprises a deposited oxide and has a thickness from about 0.5 microns to about 1.0 microns. A contact photolithography step is used to form openings 91, to expose portions of major surface 18 above source regions 33. An optional isotropic etch is used to widen openings 91 near the outer surface of layer 61 as shown in FIG. 11. Major surface 18 is then exposed to an etchant that removes material from semiconductor layer 14 to form recessed regions 99. Next, body contact regions 36 are formed through openings 91 and recessed regions 99. In one embodiment, a series of implants or a chain of implants are used so that body contact regions 36 comprise a plurality of regions as shown in FIG. 12. In one embodiment, three boron implants are used with increasing implant energies to provide the tapered shape shown in FIG. 12. That is, a higher ion implant energy provides a deeper and wider region while a lower ion implant energy provides a shallower and narrower region. By way of example, a first implant of boron at dose from about 1.0×10¹⁴ atoms/cm² to about 1.0×10¹⁵ atoms/cm² and an energy of about 200 KeV is used, then a second implant of boron at about 1.0×10¹⁴ atoms/cm² to about 1.0×10¹⁵ atoms/cm² and an energy of about 100 KeV is used, and then a third implant of boron at about 1.0×10¹⁴ atoms/cm² to about 1.0×10¹⁵ atoms/cm² and an energy of 25-30 KeV is used to form region 36. In alternative method, body contact regions 36 are formed prior the formation of dielectric layer 61 using conventional masking techniques. Dielectric layer 61 is then formed and patterned thereafter.

After body contact regions 36 are formed, source contact or conductive layer 63 is formed overlying major surface 18. By way of example, a barrier structure is formed such as titanium/titanium nitride followed by a layer comprising aluminum or an aluminum alloy. The conductive layers are then patterned using conventional photolithographic and etch techniques to form source contact layer 63 as shown in FIG. 1. In one embodiment, a final passivation layer is used overlying source contact layer 63, and comprises a deposited oxide, a deposited nitride or combinations thereof. Device 10 is then thinned, and drain contact layer 13 is formed contacting substrate 12 as shown in and further described in conjunction with FIG. 1.

FIG. 12 shows a highly enlarged partial cross-sectional view of a portion of a semiconductor device 100 having a sub-surface filled compensating trench region 222 according to another embodiment of the present invention. Device 100 similar to device 10 except that body of semiconductor material 11 comprises an n-type substrate 12, an n-type buffer layer 114 that has a lower dopant concentration than substrate 12 (e.g., about 20-35 ohm-cm) and thickness of about 10 microns to about 20 microns. Also, in device 100, sub-surface filled trench compensating region or portion 222 does not extend all the way through buffer layer 114. In this embodiment, after trenches 122 are etched, n-type dopant is introduced through the lower surface of trenches 122 to form an n+ region 223 adjacent a lower surface of trenches 122, which functions to counter-dope p-type layer 23 to electrically connect filled trench 222 to buffer epi layer 114. This embodiment is suitable for manufacturing devices of various breakdown voltages using the same filled trench process. The various breakdown voltages are then achieved using different substrate dopant concentrations and thicknesses with the depth or thickness of connecting region 223 being adjusted accordingly. Additionally, device 100 is shown without thick dielectric layer or region 431. In this embodiment, gate dielectric layer 43 separates gate conductive layer 57 from channel connecting regions 32. It is understood that thick dielectric layer 431 may be used with device 100 as well.

FIG. 13 shows a highly enlarged partial cross-sectional view of a portion of a semiconductor device 110 having a filled compensating trench region according to a further embodiment of the present invention. Device 110 is similar to device 10 except that after p-type layer 23 is deposited, that portion of p-type layer 23 along bottom portion 146 of trench 122 is removed to provide an enhanced conduction path between substrate 12 and n-type layer 26.

In summary, a new switching structure having a filled trench structure that includes a sub-surface charge compensation region and a control region overlying the compensation region. In one embodiment, a channel connect regions are used to electrically connect source regions or current carrying electrodes of the device to the sub-surface charge compensation regions when the device is in operation.

Although the invention has been described and illustrated with reference to specific embodiments thereof, it is not intended that the invention be limited to these illustrative embodiments. Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of the invention. Therefore, it is intended that this invention encompass all such variations and modifications as fall within the scope of the appended claims. 

1. A method for forming a semiconductor device comprising the steps of: providing a region of semiconductor material having a first major surface; forming a trench in the region of semiconductor material extending from the first major surface; forming a plurality of semiconductor layers within the trench including at least two opposite conductivity type semiconductor layers, and at least one buffer layer separating the at least two opposite conductivity type semiconductor layers, wherein the buffer layer has a lower dopant concentration than the at least two opposite conductivity type semiconductor layers at formation; removing portions of the plurality of semiconductor layers so that remaining portions of the plurality of semiconductor layers are recessed below the first major surface to form a sub-surface trench compensation region in a lower portion of the trench; forming a control electrode in an upper portion of the trench overlying the sub-surface trench compensation region; forming a body region within the region of semiconductor material, wherein the control electrode is configured to establish a channel in the body region when the device is in operation; and forming a source region within the body region.
 2. The method of claim 1, wherein the step of forming the plurality of semiconductor layers comprises the steps of: forming a first semiconductor layer of a first conductivity type overlying sidewall and lower surfaces of the trench; forming a first buffer layer overlying the first semiconductor layer; forming a second semiconductor layer of a second conductivity type overlying the buffer layer; forming a second buffer layer overlying the second semiconductor layer, wherein the first and second buffer layers have lower dopant concentrations than the first and second semiconductor layers at formation; and forming a dielectric layer overlying the second buffer layer.
 3. The method of claim 2 further comprising the steps of: forming a cap layer on the first semiconductor layer before forming the first buffer layer; diffusing dopants from the first semiconductor layer into the region of semiconductor material to form a second doped region of the first conductivity type; and removing the cap layer.
 4. The method of claim 1 further comprising the step of forming a channel connect region adjoining the sub-surface trench compensation region and configured to electrically couple the channel to the sub-surface trench compensation region when the device is in operation.
 5. The method of claim 1 wherein the step of forming the control electrode comprises the steps of: forming a gate dielectric layer overlying sidewalls of the trench and an upper surface of the sub-surface trench compensation region; and forming a conductive layer overlying the gate dielectric layer.
 6. The method of claim 5 further comprising the step of forming a second dielectric layer overlying the upper surface of the sub-surface trench compensation region, wherein the second dielectric layer is thicker than the gate dielectric layer.
 7. A method for forming a semiconductor device comprising the steps of: providing a region of semiconductor material having a primary surface; forming a trench extending from the primary surface; forming a charge compensated structure in a lower portion of the trench and spaced apart from the primary surface, wherein the charge compensated structure comprises a plurality of opposite conductivity layers overlying surfaces of the trench, and wherein the charge compensated structure is formed without an intervening dielectric layer between the region of semiconductor material and the plurality of opposite conductivity type layers; forming a control structure in the trench and overlying the charge compensated structure, wherein the control structure includes a control electrode for forming a channel in a body region adjacent a sidewall of the trench; forming a source region in the body region; and forming a doped region in the region of semiconductor material to electrically couple the channel to the charge compensated structure, wherein the doped region and the source region comprise the same conductivity type, and wherein the doped region overlies at least a portion of the plurality of opposite conductivity type semiconductor layers.
 8. The method of claim 7, wherein the step of forming the control structure further includes forming a first dielectric layer having a first thickness along the sidewall separating the control electrode from the region of semiconductor material, and wherein the method further includes the step of forming a second dielectric layer having a second thickness isolating the charge compensated structure from the control electrode.
 9. The method of claim 8, wherein the step of forming the second dielectric layer includes forming the second dielectric so that the second thickness is greater than the first thickness.
 10. The method of claim 7, wherein the step of forming the charge compensated structure includes the steps of: forming a first layer of a first conductivity type; and forming a second layer of a second conductivity type overlying the first layer.
 11. The method of claim 10, further comprising the step of forming a dielectric layer overlying the second layer.
 12. The method of claim 7, wherein the step of forming the control structure including forming the control structure where at least a portion of an upper surface of the control electrode is recessed below the primary surface of the region of semiconductor material.
 13. A method of forming a semiconductor device comprising the steps of: providing a region of semiconductor material having a first major surface; forming a body region in the region of semiconductor material; forming a source region in spaced relationship with the body region; forming a sub-surface trench compensated region within the region of semiconductor material, recessed below the first major surface, and comprising a plurality of opposite conductivity type semiconductor layers, wherein one of the plurality of opposite conductivity type semiconductor layers adjoins the region of semiconductor material without an intervening dielectric layer; forming a trench gate structure overlying the sub-surface trench compensated region including a gate conductive layer separated from sidewalls of the trench gate structure by a gate dielectric layer, wherein the body region and source region are in proximity to the trench gate structure, and wherein the trench gate structure is configured to control a channel in the body region when the semiconductor device is in operation; and forming a channel connecting region in the region of semiconductor material so that it is interposed between the body region and the sub-surface trench compensated region and configured to electrically couple the channel to at least one of the plurality of opposite conductivity type semiconductor layers, wherein the channel connecting region and the source region comprise a first conductivity type, and wherein the channel connecting region overlies at least a portion of the plurality of opposite conductivity type semiconductor layers.
 14. The method of claim 13, wherein the step of providing the region of semiconductor material includes providing a region of semiconductor material having a second conductivity type opposite to the first conductivity type, and wherein the step of forming the body region comprises forming the body region having the second conductivity type.
 15. The method of claim 13, wherein the step of forming the sub-surface trench compensated region includes the steps of: forming a trench extending from the first major surface; forming a first single crystal epitaxial layer of a second conductivity type overlying sidewalls and a lower surface of the trench; and forming a second single crystal epitaxial layer of the first conductivity type overlying the first layer.
 16. The method of claim 15 further comprising the step of forming a dielectric region overlying the second single crystal epitaxial layer.
 17. The method of claim 16, wherein the step of forming the dielectric region comprises the steps of: forming a thermal oxide layer overlying the second layer; forming a polysilicon layer overlying the thermal oxide layer; and forming a deposited oxide overlying the polysilicon layer.
 18. The method of claim 13 further comprising the steps of: forming a doped region extending laterally from the sub-surface trench compensated region into the region of semiconductor material and spaced apart from the first major surface, wherein the doped region and the body region comprise a second conductivity type opposite to the first conductivity type; and forming a second dielectric region separating a lower portion of the trench gate structure from the sub-surface trench compensated region, wherein the second dielectric region has a thickness greater than that of the gate dielectric layer.
 19. The method of claim 13, wherein the step of providing the region of semiconductor material comprises providing a semiconductor substrate having a semiconductor layer formed overlying the semiconductor substrate and having a lower dopant concentration than the semiconductor substrate.
 20. The method of claim 19, wherein the step of forming the sub-surface trench compensated region includes forming the sub-surface trench compensated region so that it terminates within the semiconductor layer, and wherein the method further comprises the step of forming a doped region between a lower surface of the sub-surface trench compensated region and the semiconductor substrate, wherein the doped region is configured to electrically couple the sub-surface trench compensated region to the semiconductor substrate when the device is in operation. 